Metallurgical micro-structure of liquid metal-emitter interface of field emission liquid metal ion source

Abstract

The stability of the liquid metal field-emission ion source for submicrometre lithography and ion implantation with a focused ion beam (FIB) system is of prime importance, Liquid metal ion sources (LMIS) in FIB systems have emerged as a new tool for a variety of microelectronic fabrication processes. Some micro-fabrications, such as etch rate enhancement, resist exposure, and direct microbeam ion implantation is possible because of the small virtual emitting source size, of the order of 5-60 nm [1, 2]. The basic configuration of LMIS in the FIB system consists of a wetted polycrystalline needle emitter with tip diameter in the order of 1-10 #m, and an alloy source reservoir (Fig. 1). Because of the nature of a polycrystalline emitter with grain boundaries, it is weil established that the mean jump frequency of an atom in these regions is much higher than that of an atom in the lattice [3]. Several possibilities can occur in these regions, such as liquid metal diffusion into the grain boundary (Fig. 2a) and formation of solid precipitates along the shank of the emitter tip (Fig. 2b), thus leading to source emission instability by impeding the flow of liquid metal from the reservoir to the emitter tip, causing variation in dopant concentration. Analysis of the high diffusivity path in this region, namely between two adjacent grains, is important for two reasons. Firstly, there is the question of how much these paths contribute to the calculated value of the diffusion coefficient, and, secondly by analysing the grain boundary diffusion, it is possible to approximate the diffusion coefficient in each of these regions. A more accurate solution to determine the grain boundary diffusion coefficient is to use a tracer concentration gradient as a driving force and radioactive tracer to measure the total amount of material transported. However, for the purpose of approximating the grain boundary diffusion of Pd73B27 alloy into polycrystalline Re, it is assumed that Pd is the main diffusant (because of higher Pd concentration than B) with its known self-diffusion coefficient in order to limit the com-